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What Is Larvicidal De-Worming Of Horses?

"Larvicidal de-worming" is a rather broad
term used to describe the killing of worms or parasites in the larvae
stage inside of our horses. However, the term is often intended to be
much more specific than simply killing any species of larvae at any
stage. "Larvicidal de-worming" is frequently used by horse owners to
specifically mean the killing of small strongyle larvae during the stage
of development when they are buried in the horses' intestinal wall. This
is frequently referred to as the "encysted" stage since cysts form
around the larvae.

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information found on the CowboyWay.com website, and does not accept liability
for errors or omissions. In addition, anything found on CowboyWay.com is not
intended to replace veterinary care for your horse or other animals. Always
consult a veterinarian or other trained animal health care professional
regarding any matter relating to the health and/or health care of your animals.

Life Cycle Of The Small Strongyle

The more researchers learn about encysted small
strongyles, the more reasons horse owners have to hate them. Before we go
any further, it's going to be helpful to understand the life cycle of the small strongyle:

Adult small strongyles lay eggs inside of the
horse that are passed into the horse's feces and subsequently dropped into
pastures, stalls, etc.

The eggs hatch and go through several stages of
development. During the stage when they are known as "third-stage"
larvae (L3) they are ready to be ingested by a horse. Once inside, they migrate
into the horse's intestinal wall.

When the third-stage (L3) small strongyle larvae
have entered the intestinal wall, cysts form around them. The encysted
larvae have an early stage (EL3) and a late stage (LL3) of development. While
in the early third-stage of development the larvae can remain encysted in
the horse's intestine for up to 3 years before they develop into the later
stage.

The larvae develop into a fourth stage (L4) of
development and emerge from the cysts in the intestinal wall. They then
develop into a fifth stage (L5), then adulthood where the lifecycle starts
over again.

Damage and Death

Encysted small strongyles in the horse can cause
significant inflammation, unexplained weight loss, diarrhea, poor hair coat, and
in extreme cases death. Even so, it is when the larva enter the fourth stage and
emerge from the intestinal wall that they are even more of a threat to the
horse. The damage caused by thousands or millions of these
larvae emerging at once can result in colic, diarrhea, swelling, unexplained
lethargy, sudden weight loss, and even death. Since encysted
small strongyles are not killed by typical de-worming procedures, and since they
cannot be detected by fecal analyses, even conscientious horse owners on a
regular de-worming program may be hit by surprise with encysted small strongyles.

Equine Illness After "Regular" De-Worming

Occasionally a horse will act a bit listless or as
though it doesn't feel well after it has been de-wormed (it's important to note
here that we're talking about a "regular" de-worming, not a larvicidal
de-worming as described below). In the past, it was assumed that the cause of a horse acting as
though it felt a bit off after being de-wormed was due to the results of a heavy
parasite load being killed.

However, newer research suggests there could be a much more sinister
cause. Adult small strongyles in the intestine seem to have some way of
communicating to their encysted brethren in the horse's intestinal wall. When
the adult population is sufficient, the encysted larvae remain where they are.
But when the adult population is lowered (for example, after a de-worming) the encysted, stage
three larvae (L3) are somehow prompted to develop into stage four (L4) larvae
and emerge from their encysted state. Many researchers now think that a horse
who shows signs of listlessness or illness following de-worming may be
experiencing large numbers of small strongyle larvae emerging out of their
cysts.

Larvicidal De-Worming

To kill encysted small strongyles horse owners need to use a very specific
de-worming procedure called "larvicidal de-worming." This is commonly
done by administering twice the usual dosage of fenbendazole for horses once
a day for 5 consecutive days. Fenbendazole for horses is marketed under the
names of Panacur and Safe-Guard. Currently, fenbendazole is the only FDA-approved
product to treat all stages of encysted small strongyles in horses.

Do NOT consider giving any horse, foal, or
pregnant mare a larvicidal dose of de-wormer (febendazole or any other)
without first consulting your veterinarian. To do so could result in the death
or severe illness or injury of the horse, foal, pregnant mare, or unborn foal.

Where To Buy Febendazole For Horses

As mentioned above febendazole for horses is marketed under the brand names
of Panacur and Safe-Guard. Both of these products are commonly available from
your veterinarian or local horse supply / veterinary supply store.

Panacur and Safe-Guard are both commonly sold in box sets that contain
five 1-dose syringes, 57gm each, of febendazole for horses in paste form for
oral administration.

When Is Larvicidal De-Worming Usually Done?

When or if a horse should be given a
larvicidal dose of de-wormer can vary according to the age of horse,
pregnancy status, overall health, and what is known about the horse's
medical history. For example, when some people purchase a new horse they
may not know its vaccination or de-worming history, so they feel most
comfortable giving it any vaccinations they feel it needs as well as
de-worming it, including a larvicidal de-worming. Some horse owners give
their horses larvicidal doses of de-wormers once or twice a year as a
part of their regular de-worming program. Consult your veterinarian to
see what is best for your horse.

Related Terms And Information

What is an anthelmintic?
An anthelmintic is a parasite killing drug.

What is "S. vulgaris?"
S. vulgaris is the abbreviation for large strongyles, not small strongyles. It
stands for "Strongylus vulgaris."

What is "larval cyathostomosis"?
"Cyathostomosis" is another name for small strongyles, so "larval
cyathostomosis" is referring to small strongyles in the larval state. Usually,
the term is used to refer to the encysted stage of small strongyles and the health
problems they cause. "Larval cyathostomosis" is sometimes also called
"encysted cyathostomosis."

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